Such methods for ascertaining and monitoring fill level in a container are frequently applied in the measuring devices of automation, and process control, technology. Available from Endress+Hauser are, for example, fill level measuring devices under the marks, PROSONIC, LEVELFLEX and MICROPILOT which work according to the travel time, measuring method and serve to determine and/or to monitor a fill level of a medium in a container. These fill level measuring devices transmit a periodic transmission signal in the microwave, or ultrasonic, range by means of a transmitting/receiving element in the direction of the surface of a fill substance and receive the reflected echo signals back after a distance dependent, travel time. Usually, fill level measuring devices working with microwaves can be divided basically into two classes; a first class, in the case of which microwaves sent by means of an antenna in the direction of the fill substance are reflected at the surface of the fill substance and then received back after a distance dependent, travel time, and a second class, in the case of which microwaves are guided along a waveguide in the direction of the fill substance, are reflected at the surface of the fill substance due to the impedance jump existing there and the reflected waves are then led back along the waveguide.
From the received echo signals, as a rule, an echo function representing the echo amplitudes as a function of travel time is formed, wherein each value of this echo function corresponds to the amplitude of an echo reflected at an ascertained distance from the transmission element.
In this ascertained echo function, a wanted echo is ascertained, which corresponds to the reflection of the transmission signal on the surface of the fill substance. From the travel time of the wanted echo, there results, in the case of a known propagation velocity of the transmission signals, directly the distance between the surface of the fill substance and the transmission element.
In order to simplify the echo curve evaluation, not the received, raw signal of the pulse sequence is used, but, instead, the envelope, the so called envelope curve, is ascertained. The envelope curve is won, for example, by rectifying the raw signal of the pulse sequence and then filtering with a lowpass filter.
There are a number of methods for determining the wanted echo in an envelope curve. These can be divided into two basic classes: The static ascertainment methods with static echo search algorithms; and/or the dynamic ascertainment methods with dynamic echo search algorithms, for example, by applying historical information.
In a first method of the static echo search, a static echo search algorithm is used to select, as the wanted echo, that echo having a larger amplitude than the remaining one or more echoes. Thus, that echo in the envelope curve with the largest amplitude is taken as the wanted echo.
In a second method of the static echo search, a static echo search algorithm assumes that the wanted echo is the echo in the envelope curve first occurring after the transmission pulse. Thus, the first echo in the envelope curve is selected as the wanted echo.
It is possible to combine the two methods with one another in one static echo search algorithm, e.g. by defining a so-called first echo factor. The first echo factor is a predetermined factor, by which an echo must exceed a certain amplitude, in order to be recognized as the wanted echo. Alternatively, a travel time dependent, echo threshold can be defined, which an echo must exceed, in order to be recognized as the wanted echo.
In a third method, the fill-level measuring device is told once the current fill level. The fill-level measuring device can, on the basis of the predetermined fill level, identify the associated echo as a wanted echo and follow it e.g. by a suitable dynamic, echo search algorithm. Methods of this type are referred to as echo tracking. In such a case, e.g. in each measuring cycle, maxima of the echo signal or of the echo function are ascertained and on the basis of knowledge of the fill level ascertained in the preceding measuring cycle and an application-specific maximum expected rate of change of fill level, the wanted echo is ascertained. From a travel time of the so ascertained, current wanted echo, there results, then, the new fill level.
A fourth method is described in German Patent, DE 102 60 962 A1. There, the wanted echo is ascertained on the basis of data earlier stored in a memory. In such a case, from received echo signals, echo functions are derived, which reflect the amplitudes of the echo signals as a function of their travel times. The echo functions are stored in a table, wherein each column serves for accommodating one echo function. The echo functions are stored in the columns in a sequence corresponding to fill levels associated with the respective echo functions. In operation, the wanted echo and the associated fill level are ascertained on the basis of the echo function of the current transmission signal with the assistance of the table.
In German Patent, DE 103 60 710 A1, a fifth method is described, wherein, periodically, transmission signals are sent in the direction of the fill substance, their echo signals are recorded and converted into an echo function, at least one echo characteristic of the echo function is ascertained, and, on the basis of the echo characteristics of at least one preceding measurement, a prediction is derived for the echo characteristics to be expected in the case of the current measurement. The echo characteristics of the current measurement are ascertained taking the prediction into consideration, and, on the basis of the echo characteristics, the current fill level is ascertained. This method is close to an echo tracking in the broadest sense.
In German Patent, DE 10 2004 052 110 A1, a sixth method is described, which achieves improvement of the wanted echo detection by an echo evaluation and classification of the echoes in the envelope curve.
These above described methods work, per se, without problem in a number of applications. Problems occur, however, always when the echo stemming from the fill level cannot be identified on the basis of the method without there being some doubt as to the correctness of the identification and the wanted echo signal jumps due to process conditions.
In the case of the first method, for example, measurement problems occur, when installed objects are present in the container, which reflect the transmission signals better than the surface of the fill substance.
In the case of echo tracking according to the third method, measurement problems occur, when, during operation, the wanted echo runs over a disturbance echo and, subsequently, the disturbance echo is tracked as a wrong wanted echo. Furthermore, there is a problem, when, during turn-on, the preceding wanted echo signal no longer agrees with the current one or the preceding wanted echo signal is not known.
If, mistakenly, an echo other than the fill-level echo is classified as wanted echo, there is the danger, that a wrong fill level is output, without that this is noticed. This can, depending on application, lead to an overfilling of containers, to operation of pumps empty or to other happenings connected, in part, with considerable danger.
Due to the above described measurement problems, wrong, or unsettled, measured value ascertainment of fill level of the medium in the container can occur. In the worst case, a so-called echo loss can be experienced, wherein the wanted echo signal can no longer be identified, or found.